This invention relates to a method of preparing a metal bar clad with a second metal so that the metal-metal bond is strong enough to withstand the rigors of a hot and corrosive environment. More specifically, this invention relates to a method for manufacturing an electrode for use in a magnetohydrodynamic generator comprised of a copper core clad with a corrosion-resistant, thermally and electrically conducting metal.
The principle of magnetohydrodynamic power generation utilizes heat to produce a high velocity stream of electrically conducting fluid which is then passed through a magnetic field to convert the kinetic energy of the stream into electrical energy. A typical diagonal window frame MHD power generator is an elongated duct or channel constructed of a large number of open rectangular forms or "window frames" fastened together side-by-side and insulated from each other. Around the inner perimeter of each frame are located a number of individual, generally rectangular, electrodes for collecting the electrical energy generated in each frame by the passage of the high-temperature conductive fluid. Other generator and/or electrode geometries can also be used but in each case a number of electrodes are present and separated from each other by an electrical insulator since some will act as anodes and some as cathodes as the plasma passes through the channel.
The plasma within the channel may reach temperatures of up to 2800.degree. C. resulting in electrode-plasma temperatures of up to 2100.degree. C. The plasma may be a combustion gas or inert gas seeded with a conductor such as potassium. The plasma passes through the duct at a speed which may approach or even exceed the sonic velocity. The plasma environment may be slightly oxidizing depending on the particular medium being used and its source. Thus, it is a problem to find a material from which electrodes can be made which can withstand the rigors of such an environment.
Copper has many of the characteristics required of an electrode for use in a MHD generator. It has a long service life if cooled, very stable thermal conductivity, electrical conductivity and thermal stability, and it has adequate mechanical strength characteristics. It is also inexpensive and is easily worked by common metal working techniques. However, copper can be oxidized at high temperatures and is not chemically resistant to the corrosive coal slag found in a MHD generator channel. Hot slag may condense on the colder copper surface. This solid slag layer will be colder than the plasma and therefore highly resistive. Furthermore, a low-temperature, high-resistance layer of air or corroded copper may form beneath the condensed slag, and this layer may also inhibit the passage of current.
Copper electrodes may be clad with a metal such as platinum or palladium which resists corrosion and oxidation and which has good thermal and electrical conductivity. This requires a method for manufacturing the metal-clad copper bars which will result in a very strong metal-copper bond, without oxidizing or contaminating the copper, and which will be efficient in terms of time and costs. Previously such electrodes were made by brazing or welding a metal such as sheet platinum to elongated copper bars. This method does not provide a strong reliable bond between platinum and copper and thus does not prevent oxidation of the copper layer in the MHD generator channel. This method is also tedious, expensive and time-consuming because electrodes must be manufactured individually. Alternatively, plating platinum onto copper results in a porous platinum surface which is not sufficiently resistant to corrosion. Plating is also expensive.